A gas hydrate is a solid hydrate having a structure in which a gas is trapped in a cage made of water molecules. The gas hydrate is stable under, for example, atmospheric pressure at a ten-several ° C. below zero. For this reason, its utilization has been studied as alternative means to transporting and storing natural gas in a form of liquefied natural gas (LNG). The gas hydrate can be produced under relatively easily achievable conditions of temperature and pressure, and can be stored stably as described above.
Accordingly, when natural gas extracted from a gas field is subjected to an acid-gas removal process, acid gas such as carbon dioxide (CO2) and hydrogen sulfide (H2S) is removed therefrom. Then, the natural gas is temporarily stored in a gas storage section. Thereafter, in a generating process, this natural gas is reacted with water to undergo hydration reaction, and thereby a gas hydrate is formed. This gas hydrate is in a slurry form mixed with water. In a dewatering process subsequent to the generating process, unreacted water mixed therein is removed. After undergoing a regenerating process, a cooling process and a depressurizing process, the gas hydrate is enclosed in a vessel such as a container. After that, the gas hydrate is stored in a storage unit under conditions adjusted to a predetermined temperature and pressure. As described above, the gas hydrate is in a slurry form including surplus water in the generating process. Thus, the storage or transportation of the gas hydrate without any modification requires an extra cost for that water amount. Against this problem, proposed is a natural-gas-hydrate forming method in which a slurry gas hydrate is forced to be dewatered with a screw-press dewatering system (for example, Japanese patent application Kokai publication No. 2003-105362).
Meanwhile, this screw-press dewatering system has a double structure of: an inner wall processed into a mesh form; and a case disposed outside and constituting an outer shell of the inner wall. The screw-press dewatering system removes water through the mesh-processed inner wall by forcing a slurry natural gas hydrate to move forward with a screw shaft mounted within the inner wall. Accordingly, during dewatering (condensation), a large amount of the natural gas hydrate together with water passes through the mesh holes of the inner wall, reducing the recovery rate of natural gas hydrate. Moreover, the rotating of the screw shaft in high torque incurs an additional cost. Furthermore, such high torque is developed inside the dewatering unit that is under a high pressure. Accordingly, the entire equipment is overloaded, and the screw shaft has to be sealed under conditions from high pressure to atmospheric pressure.
In order to eliminate such problems, the present inventors have proposed a gravitational dewatering method utilizing gravity, unlike the conventional forcing dewatering technique. Nevertheless, the diameters of upper and lower gravitational dewatering towers are made the same. For this reason, the following problems may occur, when there is an increase in the resistance in a dewatering zone that is above a dewatering part disposed to the gravitational dewatering tower, the dewatering part being made of a metal mesh. For example, an ejection force of the slurry pump that conveys a gas hydrate slurry to the gravitational dewatering tower is increased. Moreover, the gravitational dewatering tower is clogged by a gas hydrate. Otherwise, a liquid surface (water level) at the dewatering part is elevated, resulting in an insufficient dewatering. These problems, in some cases, make a stable operation impossible with a constant dewatering rate being maintained. Furthermore, various gas hydrate production apparatuses have been proposed so far. One of the gas hydrate production apparatuses has a double structure of an inner cylindrical container and an outer cylindrical container. The space between these containers is made as a conveying path for a formed gas hydrate (see Japanese patent application Kokai publication No. 2004-10686).
Nevertheless, in this apparatus, the outer cylindrical container is required to have a pressure-tolerable structure that does not contribute to gas hydrate formation. As a result, the size of the equipment is increased, and the cost is also increased. Moreover, the gap between the outer cylindrical container and the inner cylindrical container is filled with a gas, and problems occurs that it is difficult to remove heat of the inner cylindrical container caused by the formation of a gas hydrate, and that it is difficult to achieve efficient cooling from the outside. When the gas hydrate thus formed has a high adhesive property dependent on the degree of a percentage of water adhered to the gas hydrate, or the like, another problem occurs that the gas hydrate cannot be conveyed smoothly because the gas hydrate is stuck to a wall surface of the container.
Additionally, in FIG. 5 of the above publication, proposed is an apparatus provided with: a vertical screw conveyor formed as squeezing the top of a gas hydrate formation container; and a horizontal screw conveyor. The apparatus is to convey a formed gas hydrate. Nonetheless, this apparatus also has a problem that the gas hydrate thus formed cannot be discharged smoothly because the gas hydrate is stuck to the inner surface of the formation container.
On the other hand, according to a gas-hydrate dewatering method described in Japanese patent application Kokai publication No. 2001-342473 (Patent Document 3), firstly, a gas hydrate slurry extracted from a formation container is guided to a pressure dewatering device such as a screw press to conduct physical dewatering. Then, the gas hydrate slurry thus physically dewatered is guided and transferred to a screw conveyor, and a raw gas is incorporated thereinto. Thereby, the raw gas and water adhered to the gas hydrate are reacted with each other, and hydration dewatering is conducted. As a result, a gas hydrate having a less amount of water adhered thereto is obtained. In such a hydration dewatering method as described in Patent Document 3, a physically dewatered gas hydrate is stirred with the screw to thereby react a raw gas with water adhered to the gas hydrate, and the gas hydrate is dewatered. Nevertheless, the method has a limitation in the contacting efficiency between the water and the raw gas. Accordingly, a high dewatering rate cannot be obtained.
In contrast, considered is a fluidized-bed dewatering method. In this method, a raw gas is blown to a gas hydrate that has been subjected to physical dewatering to form a fluidized-bed. The raw gas and water adhered to the fluidized gas hydrate are reacted with each other, so that hydration dewatering is conducted. According to this method, the contacting efficiency between the water and the raw gas is high, and thereby a high dewatering rate can be obtained.
A dewatering rate hardly matters when the hydration dewatering is conducted by mechanically stirring a gas hydrate slurry that has been subjected to physical dewatering as in Patent Document 3. Nevertheless, when, for example, fluidized-bed dewatering is conducted, it is necessary to increase a dewatering rate after the physical dewatering in order to guarantee a predetermined fluid state. However, in the conventional physical dewatering, a sufficient dewatering rate cannot be obtained. As a result, there is a problem that options for hydration dewatering in a later process are limited.